1. Field of the Invention
This invention relates to microlithography, and more particularly relates to maskless lithography techniques capable of providing sub-pixel resolution from a unit-pixel spatial light modulator used as a pixel-selection device in a virtual mask system.
In our previous patent applications, we have described a maskless lithography technology using a spatial light modulator (SLM) array coupled with an imaging system and an x-y scanning stage, as shown in prior art FIG. 1. The SLM is a 2-D array of light modulators (digital micromirrors, liquid-crystal cells, etc.) each of which is programmable independently. The SLM array is illuminated by a broad-area laser beam and the hundreds of thousands of reflected beams (when the SLM elements are micromirrors) are imaged by a projection lens onto a photoresist-coated substrate mounted on a scanning stage. By programming the micromirrors with the pattern information from a Computer-Aided-Design (CAD) file, the desired image is exposed on the substrate (FIG. 2).
The above technique is further combined with an x-y scanning stage (also prior art) such as is described in FIG. 1. As the stage 6 scans, a slightly-offset different region of the substrate is illuminated by beams coming from the SLM array 3 through the projection lens 4. The pattern needed for exposure of this slightly different substrate region may be different, and is properly provided by making small changes in the pattern information fed to the SLM 3. The small changes to the information fed to the SLM 3 can be done at fast frame rates ( greater than 1 kHz), thereby accommodating scanning of the stage 6 at good speeds, thus enabling high throughputs.
A limitation of prior-art techniques is that the achievable resolution of the lithography system is limited by the single-pixel element of the spatial light modulator. For example, the typical minimum dimension (length or width) of a micromirror element is approximately 15 xcexcm. If the projection lens has a reduction ratio of 5:1, the pixel-width on the substrate (which I shall call the image resolution of the lithography system) will be:
15 xcexcm/5=3 xcexcm.
Assuming the highest-density pixel-selection unit, the image resolution of the system can be improved only by using a projection lens of higher and higher reduction ratio. For example, a 10:1 projection lens will produce a resolution of 1.5 xcexcm, and a 50:1 lens will make it possible to achieve a resolution 300 nm. Nevertheless, the fundamental limit of resolution, R, is given by:
R=dp/Mrxe2x80x83xe2x80x83Equation (1)
where dp is the size (length or width) of a pixel and Mr the reduction ratio of the projection lens, and this optical limitation cannot be overcome. This patent application, however, describes an invention that enables maskless imaging with significantly higher resolution than described by the above optical limitation, using the characteristic exposure values of the photoresist or substrate proper and a cumulative, partially overlapping voting technique with repeated sub-threshold exposures by virtual-mask offsets to select individual sub-pixel regions.
2. Description of Related Art
It is not an easy solution to achieve imaging resolution finer than the pixel size of the device as projected by the projection lens in a maskless microlithography system. The virtual mask typically is a computer-selected spatial light modulator which provides the pattern by selection and non-selection of individual pixel-selection elements. In a maskless system using a digital micromirror device as the pixel-selection device, an individual micromirror element is computer-selected (on or off) for a black or white spot typically having the same dimensions as a DMD micromirror element (1:1 projection), or a larger dimension if a magnification lens is used, or, in the best case, a smaller size if a reduction projection lens is used. A resolution improvement solution, if it is to be achieved at all, requires a finer-resolution pixel-selection device, assuming no change in the high-reduction-ratio projection lens.
It is the object of the invention to provide a novel lithography technique of maskless, seamless, small-field large-area scanning with sub-pixel resolution achieved through sub-threshold voting exposures.
Another object of the invention is to control scanning motion and laser pulses to provide sub-threshold pixel overlap capable of cumulative fractional-selection of selected sub-pixel areas with voting to achieve proper exposure with sub-pixel-selection for patterning.
Still another object of the invention is to provide greater-than-binary sub-pixel attributes by sub-pixel exposure to a variety of depths.
A feature of the invention is the use of exposure voting of cumulative, multiple, fractional-selection, sub-threshold exposures to achieve to sub-pixel image areas.
An advantage of the invention is that it eliminates the fundamental resolution limitation of lithography systems that is dictated by the pixel element size of the spatial light modulator.
Another advantage of the invention is that it may be implemented onto existing systems through computer control of sub-pixel-size overlaps.
Other objects, features and advantages of the invention will be apparent from the following written description, claims, abstract and the annexed drawings.